Log said:
If you only want a character cosplaying search "character cosplay -character_(cosplay)"
This is somewhat flawed as it inadvertently excludes images where someone else also cosplaying as this character, e.g. post #563585.
Xabid said:
Many tags become much less useful when you have a bunch of people in an image, and being able to say "this is image has Jill with red hair smiling and Jane with green hair with an open mouth" would be more useful
I've been toying around, at the back of my head, with some ideas to semantically enrich the system, whereby existing tags can be (re)used in triple form to express such semantics ( Jill <has> red_hair ) or even things like ( Keine <glomping> Mokou ), while the individual tags themselves remain searchable as well, allowing incremental deployment, backwards compatibility, and having a nice fallback to multi-tag searches ("in some undefined combination", to borrow your words), particularly to allow you to find old images that haven't been migrated to the richer semantic form. Haven't really seriously thought it out thoroughly, though. And I'm pretty sure I've seen others mention similar ideas of using ontologies...